1. Field of the Invention
The present invention relates to a piezoelectric resonance component in which damping is so performed as to improve the group delay time characteristic thereof while lowering its insertion loss.
2. Description of the Prior Art
In piezoelectric resonance components such as a piezoelectric filter and ceramic discriminators employed in an FM/PM demodulating circuit, group delay time characteristics thereof generally have been improved by mechanical damping utilizing piezoelectric materials of low Q m or elastic rubber sheets as disclosed in, e.g., Japanese Patent Laying-Open No. 20052/1980. Such mechanical damping is performed to improve phase linearity, whereas increased is equivalent resistance Re in an equivalent circuit of a piezo-resonator as shown in FIG. 2, and thus, in case of a filter, the insertion loss is increased and the signal-to-noise ratio is deteriorated as shown in FIGS. 3 and 4. FIG. 3 shows an example in which the group delay time characteristic is not improved and FIG. 4 shows an example in which the insertion loss is increased although the group delay time characteristic is improved. Such tendency is remarkable in a ladder type filter, the so-called an energy-trapping dual mode filter, three-terminal type filter utilizing a single mode of area vibration and length vibration, and the like.
Recently, a car telephone has come into wide use and the same utilizes an FM modulation system requiring a filter having an excellent group delay time characteristic and reduced insertion loss. Similar characteristics are also required in a receiving filter for AM stereo broadcasting as well as a ceramic discriminator employed in an FM/PM demodulation circuit.